Variable gain amplifiers and methods having a logarithmic gain control function

ABSTRACT

Variable gain amplifiers and methods having a logarithmic gain control function are disclosed. The variable gain amplifiers have a first inverse hyperbolic tangent stage having a current input responsive to a gain control signal, a second inverse hyperbolic tangent stage responsive to the output of the first hyperbolic tangent stage, and a Gilbert cell providing an amplifier output responsive to an amplifier input with a gain responsive to the output of the second inverse hyperbolic stage. The inverse hyperbolic gain response is altered to closely approximate the desired logarithmic characteristic by including a circuit responsive to the gain control signal to provide a current input to the first inverse hyperbolic stage which is nonlinear with respect to the gain control signal.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of variable gain amplifiers.

2. Prior Art

Amplifiers having a gain control capability are commonly used in wireless cell phone and other analog circuit applications. In the cell phone case, a signal is received on an antenna, with the strength of the received signal varying to a very large degree as the cell phone moves with respect to the base station (transmitted signal). A simple fixed gain receiver would amplify the variation to provide a widely varying signal to the analog-to-digital conversion circuitry within the cell phone that converts the signal to a digital signal for further processing. Such a wide variation in signal strength would result in an unacceptable error when converted to digital form by the analog-to-digital converter circuitry.

To avoid these problems, a variable gain amplifier is used to adjust the gain in the path of the received signal so that the full available range of the analog-to-digital converter circuitry may be used. In such applications, it is desired to provide a variable gain receive path amplification that produces a logarithmic characteristic in the gain path with a gain control signal such that the decibels of gain is linearly related to the gain control signal VAGC controlling the amplifier stage(s).

BRIEF SUMMARY OF THE INVENTION

Variable gain amplifiers and methods having a logarithmic gain control function are disclosed. The variable gain amplifiers have a first inverse hyperbolic tangent stage having a current input responsive to a gain control signal, a second inverse hyperbolic tangent stage responsive to the output of the first hyperbolic tangent stage, and a Gilbert cell providing an amplifier output responsive to an amplifier input with a gain responsive to the output of the second inverse hyperbolic stage. The inverse hyperbolic gain response is altered to closely approximate the desired logarithmic characteristic by including a circuit responsive to the gain control signal to provide a current input to the first inverse hyperbolic stage which is nonlinear with respect to the gain control signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, a circuit diagram for an embodiment of the present invention may be seen. The circuit shown has a portion identified as the linearizer and a second portion identified as the gain control, with the amplifier itself providing a differential output OUT_P, OUT_N, responsive to a differential input IN_P, IN_N and the gain control signal VAGC. The embodiment shown is suitable for amplification of RF signals, though may be used for amplification of other signals as desired.

In the circuit shown, the voltage VCS is derived in a conventional manner to establish a constant current through transistor Q₁₃ and resistor R₁₁, as well as through transistor Q₁₁ and resistor R₉, and transistor Q₁₂ and resistor R₁₀. The current through transistor Q₁₃ and resistor R₁₁ is mirrored by transistor Q₆ and resistor R₄ to transistor Q₉ and resistor R₂. Resistor R₃ has the same resistance as resistors R₂ and R₄, though resistor R₅, having a relatively low resistance, results in a current flow through resistors R₃ and R₅ and transistor Q₈, which is somewhat less than the current mirrored to transistor Q₉. Transistor Q₅, resistor R₂₁ and diode D₅ act as a beta helper for transistors Q₆, Q₈ and Q₉ to provide the base currents thereto. In addition, diode D₅ acts as a clamp, assuring turn off of transistors Q₈ and Q₉ when the voltage VCS goes to zero volts.

Similarly, the voltage VCSX3 is derived to produce a constant current in transistor Q₂₀ and resistor R₂₆, and transistor Q₁₉ and resistor R₂₅. Also, an input current ISET through transistor Q₁₅ and resistor R₂₃ is mirrored to transistor Q₂₁ and resistor R₃₄, thereby establishing the tail current for transistors Q₁₇ and Q₁₈. Transistor Q₁₆, resistor R₂₄ and diode D₆ act as a beta helper for transistor Q₂₁ to provide the base currents thereto. Also, diode D₆ acts as a clamp, assuring transistor Q₂₁ shuts off when ISET is at zero current.

In the circuit of FIG. 1, capacitors C₁ and C₂ are for noise reduction purposes. Capacitors C₃ and C₄ are for suppressing the RF signal across the current sources of transistor Q₂₀ and resistor R₂₆, and transistor Q₁₉ and resistor R₂₅.

In operation, the gain control voltage VAGC will range from approximately one VBE above ground to approximately 2.5 volts. At the low end of the range, transistors Q₁₄ and Q₇ will not significantly conduct, and accordingly, transistor Q₁₀ will also be substantially off. Consequently, the current through transistor Q₈ will all pass through transistor Q₁, establishing the voltage on the base of transistor Q₃ at one VBE above the voltage VCS. Similarly, the current through transistor Q₉ will pass through transistor Q₂, establishing the voltage on the base of transistor Q₄ at one VBE above the voltage VCS. (Diode D₁ is not sufficiently forward biased during normal operation to be conducting.) However, because the current through transistor Q₈ is slightly less than the current through transistor Q₉, the VBE of transistor Q₁ will be slightly less than that of transistor Q₂, so that transistor Q₄ will be slightly more conducting than transistor Q₃. The small differential voltage on the bases of transistors Q₃ and Q₄ results in a corresponding differential voltage on the emitters of transistors Q₃ and Q₄, providing a current in proportion thereto through resistor R₁. Since the collector currents in transistors Q₁₁ and Q₁₂ are equal, the current through resistor R₁ must be provided by a differential current through diodes D₃ and D₄, diode D₄ conducting an amount of current greater than diode D₃ equal to twice the current through resistor R₁. Consequently, the differential output of the linearizer will be slightly biased by the presence of resistor R₄ so that a minimum value of the differential control CONTROL_P, CONTROL_N is provided to the amplifier. The common mode voltage of the differential control CONTROL_P, CONTROL_N is set by resistor R₁₉ and diodes D₃ and D₄, the current there through being equal to the sum of the current of current sources Q₁₁ and R₉, and Q₁₂ and R₁₀, and thus independent of the gain control voltage VAGC. Also, diodes D₃ and D₄ could be realized as diode connected transistors, both diodes and diode connected transistors being included within the meaning of the word diode as used herein and in the claims to follow.

As the gain control voltage VAGC is increased, commanding greater gain of the amplifier, transistor Q₁₄ will begin to turn on, with the current through transistor Q₁₄ being mirrored to transistor Q₁₀ and resistor R₉. If resistor R₁₃, an optional resistor, is present, there will also be a component of current through resistors R₁₃ and R₁₂ and transistors Q₁₄ and Q₇. As the gain control voltage VAGC continues to increase, the voltage drop across resistor R₁₃, if present, will reach the forward conduction diode voltage drop of diode D₂. If resistor R₁₃ is not present, the voltage drop across resistor R₆ will reach the forward conduction voltage drop of diode D₂, in either case providing a more steeply increasing current through resistor R₁₂ with further increasing gain control voltage VAGC. Accordingly, for the minimum value of voltage VAGC, the current through transistor Q₁₀ will be substantially zero. With initially increasing values of VAGC, there will be an approximately proportional increase in current through transistor Q₁₀, and with increasing values of VAGC in the upper portion of the VAGC range, the current through transistor Q₁₀ will increase at a greater rate because of the conduction through diode D₂ also contributing to the current flow through transistors Q₁₄ and Q₇.

The current through transistor Q₁₀ diminishes the current through transistor Q₁, by providing an alternate path for the current through transistor Q₈, thereby further raising the voltage on the base of transistor Q₄, thereby lowering the voltage on the emitter of transistor Q₃ with respect to the emitter of transistor Q₄, increasing the current flow through resistor R₁ to further increase the current flow through diode D₄ and further decrease the current flow through diode D₃, increasing the differential voltage CONTROL_P, CONTROL_N on the output of the linearizer.

The differential input IN_P, IN_N is applied to the bases of transistors Q₁₇ and Q₁₈. The common mode voltage for the differential input, as well as the input impedance thereof, is set by currents through resistors R₃₀ and R₂₇ and resistors R₃₂ and R₃₁ by current sources Q₂₀ and R₂₆ and Q₁₉ and R₂₅, respectively. The differential input IN_P, IN_N determines the division of the tail current through transistors Q₂₁ and R₃₄ between transistors Q₁₇ and Q₁₈. When the differential input is zero, the tail current will divide equally between transistors Q₁₇ and Q₁₈. For a positive differential input, transistor Q₁₇ will conduct a larger portion of the tail current and transistor Q₂₈ a smaller portion of the tail current, and for a negative differential input, transistor Q₁₇ will conduct a smaller portion of the tail current and transistor Q₂₈ a larger portion of the tail current. Assuming for the moment that the gain control voltage VAGC is at its lower limit, the voltage on CONTROL_P will be slightly higher than the voltage on CONTROL_N. Thus, transistors Q₂₄ and Q₂₃ will be conducting slightly more than transistors Q₂₂ and Q₂₅. However, transistors Q₂₂ through Q₂₅ are all the same size. Consequently, since the current through resistor R₃₃ is equal to the sum of the currents through transistors Q₂₂ and Q₂₃, and the current through resistor R₂₂ is equal to the sum of the currents through transistors Q₂₄ and Q₂₅, the currents through resistors R₂₂ and R₃₃ will be equal, so that the differential output OUT_P, OUT_N will be zero.

With a positive differential signal input IN_P, IN_N, a larger fraction of the tail current through transistors Q₂₁ and R₃₄ will flow through transistor Q₁₇ and less through transistor Q₁₈. The division of current between transistors Q₂₃ and Q₂₅ will be the same as the division of current between transistors Q₂₄ and Q₂₂. However the larger current through transistor Q₁₇ under these conditions, will cause a larger increase in the current in transistor Q₂₄ than in transistor Q₂₃ and a larger decrease in current through transistor Q₂₂ than in transistor Q₂₅. Consequently, the increase in current through transistor Q₂₄ will more than offset the decrease in current through transistor Q₂₅, increasing the current through resistor R₂₂ to reduce the voltage on the output terminal OUT_N. Similarly, the decrease in current through transistor Q₂₂ will be more than the increase in current through transistor Q₂₃, thereby decreasing the total current through resistor R₃₃ to increase the output on output terminal OUT_P. For higher differential inputs IN_P, IN_N, more of the tail current for transistors Q₁₇ and Q₁₈ will be steered through transistor Q₁₇ and less through transistor Q₁₈, increasing the differential output OUT_P, OUT_N accordingly.

For higher values of the gain control voltage VAGC, the differential voltage CONTROL_P, CONTROL_N will increase, as previously described. This steers a greater fraction of the current through transistor Q₁₇ through transistor Q₂₄ and less through transistor Q₂₂, and a greater fraction of the current through transistor Q₁₇ through transistor Q₂₃ and less through transistor Q₂₅, having the net effect of increasing the gain of the amplifier.

Having now described the detailed operation of the circuit of FIG. 1, the general characteristics thereof will now be apparent. Transistors Q₂₂, Q₂₄, Q₂₃, Q₂₅, Q₁₇, Q₁₈, Q₂₀, Q₁₉, Q₁₅, Q₁₆ and Q₂₁ and the other circuit elements associated with those transistors form a Gilbert cell having a characteristic hyperbolic tangent function non-linearity. However, transistors Q₁, and Q₂ provide an inverse hyperbolic tangent stage and diodes D₃ and D₄ provide another inverse hyperbolic tangent stage, leaving a net single inverse hyperbolic tangent function for gain versus the AGC signal VAGC. This net inverse hyperbolic tangent function coarsely approaches the desired logarithmic gain control function, which is forced to closely approach the desired logarithmic gain control function by the shaping effect of diode D₂ and resistor R₁₂ and the circuitry associated therewith (and resistor R₁₃, if the same is included). Finally, of course, the gain control circuit is biased to operate in a single quadrant by resistor R₅ and the circuitry associated therewith, resistor R₅ assuring that the current through transistor Q₁ is less than the current through transistor Q₂, even with transistor Q₁₀ turned off.

In the embodiment shown, if the gain control voltage VAGC was applied to the circuit so as to provide a differential voltage on the bases of transistors Q₃ and Q₄ proportional to the gain control voltage, the current through resistor R₁ and the direction thereof would be linearly dependent on the gain control voltage VAGC. That current would provide a differential voltage drop across diodes D₃ and D₄ having the characteristic of an inverse hyperbolic function, so that that differential voltage drop applied to the bases of transistors Q₂₄ and Q₂₂, and transistors Q₂₃ and Q₂₅, would result in a division of current between each of those transistor pairs, linearly proportional to the gain control voltage. However, the differential voltage on the bases of transistors Q₃ and Q₄ is not linearly proportional to the gain control voltage VAGC, but rather, neglecting diode D₂ for the moment, is dependent upon the differential base emitter voltages of transistors Q₁ and Q₂ having a current which is approximately proportional to the gain control voltage, thereby providing a further inverse hyperbolic tangent function on the gain in response to the automatic gain control voltage VAGC. Diode D₂ and the circuitry associated therewith makes the differential current in transistors Q₁ and Q₂ non-linearly related to the gain control voltage VAGC in a manner to alter the gain control characteristics to achieve more precise matching of the desired logarithmic gain control characteristic. Thus, the circuit includes a first inverse hyperbolic stage having a current input responsive to a gain control signal, a second inverse hyperbolic stage responsive to the output of the first hyperbolic stage, and a Gilbert cell providing an amplifier output responsive to an amplifier input, with a gain responsive to the output of the second inverse hyperbolic stage. The matching of the gain control with the desired logarithmic gain control function is improved by providing a current input to the first inverse hyperbolic stage which is non-linear with respect to the gain control signal.

While a specific embodiment of the present invention has been disclosed and described herein, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention. 

What is claimed is:
 1. A variable gain amplifier comprising: a first inverse hyperbolic stage having a current input responsive to a gain control signal; a second inverse hyperbolic stage responsive to the output of the first hyperbolic stage; and, a Gilbert cell providing an amplifier output responsive to an amplifier input with a gain responsive to the output of the second inverse hyperbolic stage.
 2. The variable gain amplifier of claim 1 further comprised of a circuit responsive to the gain control signal to provide a current input to the first inverse hyperbolic stage which is nonlinear with respect to the gain control signal.
 3. A variable gain amplifier comprising: first, second, third and fourth transistors, each having an emitter, a base and a collector; the first and second transistors having their bases coupled together; a first circuit providing a differential current through the first and second transistors responsive to an automatic gain control signal; first and second current sources coupled to the emitters of the third and fourth transistors, respectively, and a first resistor coupled between the emitters of the third and fourth transistors; the emitters of the first and second transistors being coupled to the bases of the third and fourth transistors, respectively; the collectors of the third and fourth transistors being coupled to first and second diodes and to the control of a Gilbert cell.
 4. The variable gain amplifier of claim 3 wherein the first circuit provides a differential current through the first and second transistors which is nonlinearly responsive to the automatic gain control signal.
 5. The variable gain amplifier of claim 4 wherein the first circuit provides a differential current through the first and second transistors which is biased to have the same polarity throughout the operating range of the automatic gain control signal.
 6. The variable gain amplifier of claim 5 wherein the first circuit comprises first and second current sources providing first and second currents to the first and second transistors, respectively, the first current source providing a smaller current than the second current source, the first circuit diverting part of the current of the first current source from the first transistor responsive to the automatic gain control signal.
 7. The variable gain amplifier of claim 5 wherein the Gilbert cell has a differential input and a differential output. 